Kofron et al U.S. Pat. No. 4,439,520 discloses that multicolor photographic elements of improved speed-granularity relationship, minus blue to blue speed separation, and sharpness can be achieved by employing in one or more of the image recording layers a chemically and spectrally sensitized high aspect ratio tabular grain silver bromide or bromoiodide emulsion. In such an emulsion at least 50 percent of the total projected area of the grains is provided by tabular grains having a thickness of less than 0.3 .mu.m, a diameter of at least 0.6 .mu.m, and an average aspect ratio greater than 8:1. Kofron et al indicates that preferred high aspect ratio tabular grain emulsions are those having an average diameter of at least 1.0 .mu.m, most preferably at least 2.0 .mu.m. Kofron et al states that both improved speed and sharpness are attainable as average grain diameters are increased.
While the high aspect ratio tabular grain emulsions disclosed by Kofron et al produce excellent multicolor photographic elements of higher photographic speeds, it is for some photographic uses more desirable to reduce granularity to minimal levels. Within limits granularity can be reduced by simply coating more silver halide grains per unit area, referred to as increasing silver coverages. Unfortunately, this results in loss of image sharpness and inefficient use of silver. Holding the silver coverage constant, it is conventional practice to improve granularity by reducing mean grain size. Photographic speed is reduced as a direct function of reduced grain size.
While Kofron et al is aware that granularity can be improved at the expense of photographic speed, there is a bias in the art against reducing the mean diameter of tabular grain emulsions to an extent sufficient to optimize granularity for photographic elements of moderate and lower camera speeds. First, the Kofron et al teaching of tabular grain diameters of at least 0.6 .mu.m is not compatible with efficient use of silver at moderate and lower camera speeds. Second, in suggesting that sharpness increases with increasing grain diameters in high aspect tabular grain emulsions, Kofron et al necessarily suggests that reducing grain diameters in these emulsions will reduce sharpness.
The art has long recognized that visible light is more highly scattered by smaller silver halide grain diameters. Berry, "Turbidity of Monodisperse Suspensions of AgBr", Journal of the Optical Society of America, Vol. 52, No. 8, August 1962, pp. 888-895, examined monodisperse silver bromide emulsions of mean grain sizes in the range of from 0.1 to 1.0 .mu.m at wavelengths of from 300 to 700 nm and found general agreement with theoretical predictions of light scattering. Ueda U.S. Pat. No. 4,229,525 states that when silver halide grain diameters approximate the wavelength of exposing radiation, increased scattering of light by the grains occurs with concomittant losses in sharpness. Locker et al U.S. Pat. No. 3,989,527 states that silver halide grains having a diameter of 0.2 .mu.m exhibit maximum scattering of 400 nm light while silver halide grains having a diameter of 0.6 .mu.m exhibit maximum scattering of 700 nm light. From interpolation of Locker et al it is suggested that silver halide grains in the range of from 0.4 to 0.55 .mu.m in diameter exhibit maximum scattering of light of from about 550 to 650 nm. Thus, the suggestion by Kofron et al of tabular grains of at least 0.6 .mu.m in diameter avoids what are generally recognized to be grain sizes of maximum light scatter in the minus blue portion of the visible spectrum--that is, the green and red portions of the visible spectrum.
There is precedent in the art for taking the known light scattering properties of silver halide grains into account in selecting grain sizes for multicolor photographic elements. Zwick U.S. Pat. No. 3,402,046 discusses obtaining crisp, sharp images in a green sensitive emulsion layer of a multicolor photographic element. The green sensitive emulsion layer lies beneath a blue sensitive emulsion layer, and this relationship accounts for a loss in sharpness attributable to the green sensitive emulsion layer. Zwick reduces light scattering by employing in the overlying blue sensitive emulsion layer silver halide grains which are at least 0.7 .mu.m, preferably 0.7 to 1.5 .mu.m, in average diameter.
Wilgus et al U.S. Pat. No. 4,434,226; Solberg et al U.S. Pat. No. 4,433,048; Jones et al U.S. Pat. No. 4,478,929; Maskasky U.S. Pat. No. 4,435,501; and Research Disclosure, Vol. 225, January 1983, Item 22534, are considered cumulative with the teachings of Kofron et al. The optical transmission and reflection of tabular grain emulsions as a function of tabular grain thicknesses in the range of from 0.07 to 0.16 .mu.m is described in Research Disclosure, Vol. 253, May 1985, Item 25330. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
Tabular grain emulsions having mean grain diameters of less than 0.55 .mu.m are known in the art. Such tabular grain emulsions have not, however, exhibited high aspect ratios, since achieving high aspect ratios at a mean grain diameter of less than 0.55 .mu.m requires exceedingly thin grains, less than 0.07 .mu.m in thickness. Typically tabular grains of smaller mean diameter are relatively thick and of low average aspect ratios. A notable exception is Reeves U.S. Pat. No. 4,435,499, which discloses the use of thin (less than 0.3 .mu.m in thickness) tabular grain emulsions in photothermography. Preferred tabular grain emulsions are disclosed to have average grain thicknesses in the range of from 0.03 to 0.07 .mu.m and to have average aspect ratios in the range of from 5:1 to 15:1.
A tabular grain emulsion exhibiting a mean diameter of less than 0.55 .mu.m known to have been incorporated in a multicolor photographic element is Emulsion TC16, reported and compared in the examples below. Emulsion TC16 exhibits a mean grain diameter of 0.32 .mu.m, a mean grain thickness of 0.06 .mu.m, and an average tabular grain aspect ratio of 5.5:1. Emulsion TC16 has been employed in a blue recording yellow dye image providing layer unit overlying green and red recording dye image provide layer units. In the blue recording layer unit in addition to Emulsion TC16 was an overlying high aspect ratio tabular grain emulsion layer having a mean tabular grain diameter of 0.64 .mu.m, satisfying the requirements of Kofron et al, and, over these emulsion layers, a still faster blue recording emulsion comprised of tabular grains having a mean tabular grain diameter of 1.5 .mu.m also satisfying the requirements of Kofron et al.
Daubendiek et al U.S. Ser. No. 790,693, filed Oct. 23, 1985, now abandoned in favor of continuation-in-part U.S. Ser. No. 891,804, filed Aug. 1, 1986, discloses a layer order arrangement in which at least one reduced diameter high aspect ratio tabular grain emulsion layer comprised of silver bromide or bromoiodide grains having a mean diameter in the range of from 0.2 to 0.55 .mu.m including tabular grains having an aspect ratio of greater than 8:1 accounting for at least 50 percent of the total projected area of the grains overlies a blue recording emulsion layer.